PREFACE vii 



and species are included under a synoptic key which comes at the 

 close of each chapter except as noted above; in some cases it is 

 carried to species but in others only to genera. The form utilized 

 for the keys has been in constant use for many years at the Uni- 

 versity of Illinois, having been applied to many aquatic types by 

 Professor S. A. Forbes and his associates. The introductory num- 

 ber of each key line is followed by an alternative number printed 

 in parentheses and on reaching a decision that this line is not ac- 

 ceptable, the student proceeds at once to the line introduced by 

 the alternative number; in case a given alternative is accepted the 

 further course of the inquiry is indicated by a number at the close 

 of the line. 



In order to achieve maximum ease in use and perspicacity in 

 grasping the facts presented, all the information on a given form, 

 viz., the illustration, the description, and the biological features with 

 the frequence, range, and other special data, are included between 

 the key line which introduces the name and the key line next fol- 

 lowing. The total information on a single type forms thus a solid 

 panel and appeals promptly and as a whole to the eye and mind of 

 the student. Each chapter closes with a brief list of the most 

 essential references to the topic. No textbooks are cited and only 

 such works are noted as may be considered indispensable for pres- 

 ent-day study of North American forms. The student is cautioned 

 not to regard any such list as in any sense a bibliography of the 

 subject. 



To encompass such a mass of material within the limits of a 

 single volume, even tho it be generous in size, has necessitated 

 brevity of treatment at every point. Technical terms are defined 

 or discussed only once and no glossary is introduced. The index 

 includes important terms and all of the scientific names used in 

 the keys so that the reader can find every item promptly. 



A serious effort was made to attain uniformity in the use of 

 names thruout the entire work but the worker will find that this 

 end was not fully achieved. The most conspicuous failure in this 

 particular obtains in the citation of host names for various para- 

 sitic species. In all such cases that name is employed which was 

 used by the authority from which the record is cited. It was felt 



